1. Field of the Invention
The present invention relates to a fixing device and an image forming apparatus, such as a copier, a printer, a facsimile machine, a multifunctional device, etc., with the fixing device.
2. Description of the Related Art
As used in various types of image forming apparatuses, a fixing device of a thin fixing belt type consisting of an elastic rubber layer and a metal substrate or the like is known. In such a system, due to the low-heat capacity of the thin fixing belt, the energy needed in heating the fixing belt is significantly reduced and a warm-up time period from when power source is turned on, for example, to when room temperature is increased to a prescribed printable level (i.e., a reload temperature level) can be shortened. Further, a time to first print (i.e., a time period from receiving a printing request to completing sheet ejection through preparing and conducting the printing) is shortened as well.
In a conventional fixing apparatus as described in Japanese Patent Application Publication No. 2007-233011 (JP-2007-233011-A), a nip is formed by a pressing roller, an endless belt as a hollow endless rotary body, a pressing roller in contact with an outer surface of the endless belt, and a nip formation member placed inside the endless belt, with the nip formation member and the pressing roller sandwiching the endless belt. On an inner circumferential side of the endless belt, there is provided a single heat source as a heating device for heating the entire width of the endless belt with radiant heat. Since the endless belt can be directly heated by the radiant heat from the heat source at places other than that of the position of the nip formation member, efficiency of heat transfer from the heat source to the endless belt is widely improved. As a result, power consumption and the above-described time to first print can be further reduced.
In such a conventional system, in a sheet passage region on the endless belt over which the sheet passes, heat is deprived therefrom by the sheet contacting the endless belt as described in JP-2007-233011-A. Specifically, when since heat capacity of the endless belt is relatively small, heat transferred to the endless belt is deprived by a sheet, and accordingly the temperature of the endless belt drops significantly in the sheet passage region in the conventional fixing apparatus. Therefore, when sheets are continuously fed, defective fixing occurs unless the sheet passage region on the endless belt is heated by the heat source up to a prescribed fixing temperature capable of maintaining an appropriate fixing condition.
Further, in a fixing device installed in an image forming apparatus that is capable of feeding relatively small- and large-sized sheets and obtaining higher productivity of the small-sized sheet, an endless belt more likely loses and cannot obtain sufficient heat from a heat source when the small size sheet is fed and printed than the large size sheet. Therefore, the sheet passage region of the endless belt becomes unable to maintain a prescribed fixing temperature resulting in defective fixing when the small-sized sheets are continuously fed and printed.
To resolve such a problem, conceivably, power supplied to the heater to heat 1 the endless belt when the small-sized sheet is printed can be increased beyond what is supplied when the large-sized sheet is printed. However, in general, the amount of power available to the fixing device is limited depending on the power consumed by the other devices in an image forming apparatus to keep the total power consumption within certain limits. Thus, if power to be supplied to the heat source is simply increased to execute printing of the small-sized sheet, power supplied to the heat source likely reaches the maximum level available to the fixing device before the sheet passage region of the endless belt is heated up to the prescribed fixing temperature. As a result, the heat source is unable to receive enough power, and consequently the fixing device falls into lack of power. Consequently, the heat source can no longer heat the sheet passage region of the endless belt up to the prescribed fixing temperature.